Resistance Testing-Planing Hull Sailboat

Discussion in 'Boat Design' started by Doug Lord, May 4, 2010.

  1. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    The hull I want to test is almost 18' long and will be tested at different weights. The boat will use a small hydrofoil on the daggerboard and one on the rudder. The point of the testing is to optimize the size of these foils to assure that they actually benefit the design more than they hurt it. The foils are there to improve speed and handling-not to fully fly the boat.( "foil assist") I'm associated with a marine testing laboratory that will help-they'll set up the data collection so that resistance can be accurately measured and referenced exactly to speed and set up to be easily analyzed.
    The lab has not done this kind of tow testing before but they have a wide background in testing to various standards of almost every marine component-mostly powerboat components,tanks, bilge pumps, wheels, hydraulics ect.
    My proposal is to attach a tow line to a mast stub with the attachment point at the CE of the rig and test between 0 and 20 knots with the boat level(athwartship).
    Foils would be tested at different angles of incidence according to a plan that we will develop. The boat will be tested with a "plain" daggerboard and rudder and then with the foil equipped boards.
    Some questions:
    1) should the towing be done outside the wake of the towboat-will that turbulence affect the results?
    2) seems to me that initial testing should be done in flat calm water-no waves?
    3) should the tow line from the boat be kept relatively level instead of going at a down angle to the towboat?
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    I would appreciate any suggestions or comments particularly if you have testing experience. Thanks!

    picture of model of boat to be tested:
     

    Attached Files:

  2. jwboatdesigns
    Joined: Apr 2010
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    Location: Hamilton New Zealand

    jwboatdesigns John Welsford

    Ive done some testing on a hull of about this size and proportions, it was a foil stabilised type but my students ( marine design degree, third year) had a ball testing it with all sorts of lift and stabilising foils.

    Tow it in undisturbed waters, dead flat water, keep the towline as close to parallel with the waters surface as possible to minimise trim changes, it may help to radio control the rudder so it can be made to track properly.

    Hope that helps

    John Welsford


     
  3. daiquiri
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Doug, are you testing a full-size boat or a scale model?
     
  4. Doug Lord
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    Doug Lord Flight Ready

    ============
    Thanks, John! I think I'll ride the thing so I can adjust the foils-if I'm not too scared. It needs the weight anyway. I appreciate your comments very much. Crew weight the same-wing ballast changed....
     
  5. Doug Lord
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    Doug Lord Flight Ready

    ==================
    Slavi, the full size boat.......
     
  6. jehardiman
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    jehardiman Senior Member

    For something that size there is always NSWCCD.... anyway...

    When high speed models are tested, there is usualy a need for a cricket or two point dyno to constrain the model in yaw as there is no active control to prevent kiteing. On an 18' model yaw may be considerable. A two point dyno will also help in determining the directional stability derivatives.
     
  7. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ===========
    As it stands now,I'll be in it steering it -just as I would be under sail.....
    PS-what is "NSWCCD" ? Naval Surface Warfare Center CD???
     
  8. jehardiman
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    As you think: Naval Surface Warfare Center Carderock Division...The David Taylor Model Basin for us old folks. The "slow, deep" tank carriage will do 20 knots, the "high speed" basin will do 50. The models are about the size of your hull.

     
  9. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    1. Keep towing line length at least 4 times longer than length of tug boat - this is important to minimize effect of tug's propeller and wake flow.
    2. Towing point - it is good to have it at same level as CE of sailing rig;
    3. If the model/boat is asymmetrical (if it is stabilized on one side), it will yaw(drift) to one side, so towing rope will not be parralel to tug's CL.
     

  10. Doug Lord
    Joined: May 2009
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    Location: Cocoa, Florida

    Doug Lord Flight Ready

    ==========
    Thanks, Alik! The boat is symetrical and a full size working prototype.
     
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